Japan’s nuclear regulator says the operator of a damaged nuclear plant knew it might be hit by a far bigger tsunami than it was designed to withstand.

The Nuclear and Industrial Safety Agency says the operator informed it just four days before Japan’s massive March 11 earthquake and tsunami that waves exceeding 10 meters (33 feet) could hit the Fukushima Dai-ichi nuclear plant.

The plant was only designed to withstand a tsunami about half that height.

Agency officials said Wednesday they recommended that Tokyo Electric Power Co. take measures to prepare for a bigger tsunami but did not give specific instructions.

The March tsunami hit the plant with waves bigger than 15 meters (49 feet), even higher than the operator’s estimate.